In skid control, the slip rate is calculated from the calculated wheel speed and when the calculated slip rate reaches a predetermined value, the brakes are released, while when the slip rate is restored to another preset value, the braking oil pressure is reapplied. The time during which the brakes are released is measured and the next instant of brake release is controlled on the basis of the result of the measurement. Repeating this series of operations thereafter, the frictional coefficient between the wheels and the road surface is kept at a maximum value so that the stopping distance is shortened.
The calculation of the wheel speed is one of the most important factors necessary for skid control as a whole and therefore must be processed exactly and swiftly.
To calculate the wheel speed from the signal delivered by the wheel speed sensor, there are two methods such as a proposed method in which pulses from the wheel speed sensor are counted for a predetermined constant time and a method in which the time interval between adjacent pulses is measured. The former method is not adapted for an anti-skid apparatus which requires rapid calculations, since this method needs to count pulses and therefore requires a certain time. The latter method, which measures the interval between adjacent pulses, can perform a rapid processing since only a time equal to the pulse-to-pulse period is required in this case. However, the measurement of only the pulse-to-pulse period results in a rather large error and therefore in practical applications it is necessary to measure several numbers of such periods and to figure out the average of them. Accordingly, it becomes difficult also in this case to complete the processing in a very short time.
The pulse-to-pulse periods are successively stored in a memory (RAM) and they are read out for processing in the case of the wheel speed being calculated. At the time of calculating the wheel speed, it is necessary to check which data block is to be used. According to methods currently adopted, the program for calculation is very complicated so that the number of locations in memory used for calculation processing is considerable.